Usually, when coating molded metal products such as automobile bodies, metal parts for motorcycles, household electric appliances, and steel furniture, an electrodeposition coating composition is first applied and baked; an intermediate coating composition is applied to the baked electrodeposition coating film and baked; a colored aqueous coating composition is applied; preheating is performed; and then a clear coating composition is applied and baked, forming a multilayer coating film (a coating film formation method utilizing a so-called “3C2B process”). By such baking, uneven coating films are smoothed, and a multilayer coating film with excellent corrosion resistance and excellent finish can be obtained.
However, when baking is performed after application of each coating composition, not only is a great energy cost needed for baking, but immense labor and expenditure are also needed for operation and maintenance of the baking facilities. Further, for the purpose of reducing low-volatile organic compounds in coating compositions (VOC reduction), organic solvent-based coating compositions have been shifted to aqueous coating compositions.
For the purposes of energy-saving, omission of processes, and VOC reduction, a multilayer coating film with excellent corrosion resistance and excellent finish obtained by a coating film formation method (hereinafter, may be abbreviated as an “aqueous 3C1B process”) in which a first colored aqueous coating composition, a second colored aqueous coating composition, and a clear coating composition are sequentially applied to an electrodeposition coating film, and the three layers are simultaneously cured by heating, is desired.
Conventionally, for improving finish in a 3-coat 1-bake process, Patent Document 1, for example, discloses a method for forming a high-quality multilayer coating film utilizing a 3-coat 1-bake process, in which: an organic solvent-based coating composition (referred to as a non-aqueous dispersion coating composition in the specification) is used as at least one of an intermediate coating composition, a top coat base coating composition, and a top coat clear coating composition; and, before baking, these coating compositions are sequentially applied to a laser-textured dull steel plate that has been undercoated by cationic electrodeposition coating, followed by baking. However, the method has a problem in that, in order to form a high-quality three-layer coating film in a single baking, a molded product formed from a costly laser-textured dull steel plate must be used to ensure the desired finish in the vertical section and horizontal section.
Patent Document 2 discloses a method for forming a multilayer coating film, in which: electrodeposition coating composition (A) is applied to an object to be coated; the coating film is cured by heating; and intermediate coating composition (B), top coat base coating composition (C), and clear coating composition (D) are applied wet-on-wet, and heated to simultaneously cure the three coating films. In the method, a cationic electrodeposition coating composition that contains: a resin (a) obtained by reacting a cyclic ester compound with hydroxyl in a bisphenol epoxy resin; vinyl resin (b) having a solubility parameter value of less than 9.6; polyalkylene glycol (c) having a solubility parameter value of less than 9.6; and pigment component (d) whose average particle diameter is adjusted to be not greater than 0.5 μm, is used as electrodeposition coating composition (A). However, intermediate coating composition (B) and top coat base coating composition (C), which are used in the multilayer coating film formation method, are organic solvent-based coating compositions, and are not aqueous coating compositions that can achieve VOC reduction.
Patent Document 3 discloses a multilayer coating film formation method in which: a cationic electrodeposition coating composition is applied to a base material to form a cured electrodeposition coating film with a glass transition temperature of not less than 110° C. and a surface roughness (Ra) of not greater than 0.3 μm; an intermediate coating composition, a top coat base coating composition, and a top coat clear coating composition are sequentially applied to the surface of the electrodeposition coating film to form three coating films, namely, an uncured intermediate coating film, an uncured top coat base coating film, and an uncured top coat clear coating film; and the three coating films are simultaneously cured by heating. However, when only the glass transition temperature and the surface roughness (Ra) of the cured coating film of the cationic electrodeposition coating composition are regulated, a multilayer coating film formed on the cationic electrodeposition coating film through an aqueous 3C1B process could not achieve both the desired corrosion resistance and the desired finish at the same time.
Patent Document 4 discloses a multilayer coating film formation method in which: a cationic electrodeposition coating composition is applied to a base material to form a cured electrodeposition coating film with an absorption rate of toluene, which is a nonpolar organic solvent, of not greater than 25%; an intermediate coating composition, a top coat base coating composition, and a top coat clear coating composition are sequentially applied to the surface of the cured electrodeposition coating film to form three coating films, namely, an uncured intermediate coating film, an uncured top coat base coating film, and an uncured top coat clear coating film; and the three coating films are simultaneously cured by heating. However, the multilayer coating film formation method of Patent Document 4 is intended to apply organic solvent-based coating compositions on an electrodeposition coating film in an overlapping manner.
Patent Document 5 discloses a coating film formation method that includes: a process of applying an aqueous intermediate coating composition to a cationic electrodeposition coating film to form an uncured aqueous intermediate coating film; a process of applying an aqueous base coating composition to the uncured aqueous intermediate coating film to form an uncured base coating film; a process of applying a clear coating composition to the uncured base coating film; and a process of simultaneously curing by heating the uncured aqueous intermediate coating film, the uncured aqueous base coating film, and the uncured clear coating film. In this method, the metal ion concentration and the amount of neutralizer in the cationic electrodeposition coating composition are specified.
However, during preheating or heat-drying the cationic electrodeposition coating film described in the Patent Document 5, the electrodeposition coating film swells, resulting in undulation and unevenness. When a multilayer coating film is formed through an aqueous 3C1B process, the multilayer coating film cannot achieve both the desired corrosion resistance and the desired finish at the same time due to the unevenness of the electrodeposition coating film.
Patent Document 6 discloses a multilayer coating film formation method in which: an intermediate coating composition, a top coat base coating composition, and a top coat clear coating composition are applied to a cured cationic electrodeposition coating film; and the uncured coating films of these coating compositions are simultaneously cured by baking. The cured electrodeposition coating film has a center line mean roughness (Ra) in a roughness curve of 0.05 to 0.25 μm, and a center line mean roughness (Pa) in a profile curve of 0.05 to 0.30
However, when only the center line mean roughness (Ra) in the roughness curve and the center line mean roughness (Pa) in the profile curve of the cured coating film of the cationic electrodeposition coating composition are defined to be 0.05 to 0.25 μm and 0.05 to 0.30 respectively, the desired corrosion resistance and the desired finish are not obtained through a 3-coat 1-bake process in which a first colored aqueous coating composition, a second colored aqueous coating composition, and a clear coating composition are applied to an electrodeposition coating film in an overlapping manner.
Patent Document 7 discloses a method for forming a multilayer coating film on an electrodeposition coating film through a 3-coat 1-bake process, wherein the electrodeposition coating film before curing by heating has an arithmetic average roughness (Ra) in a roughness curve of 0.3 to 4.0 μm. However, even when the arithmetic average roughness (Ra) in the roughness curve of the uncured electrodeposition coating film is defined, a multilayer coating film that is obtained through a 3-coat 1-bake process cannot achieve both the desired corrosion resistance and the desired finish at the same time, just as in the case of an electrodeposition coating film with a surface roughness (Ra) of not greater than 0.3 μm, which is described in Patent Document 3.
Patent Document 8 discloses a multilayer coating film formation method in which: first colored coating composition (B), second colored coating composition (C), and clear coating composition (D) are sequentially applied wet-on-wet on a cured coating film of electrodeposition coating composition (A) that has a loss on heating (X) of not greater than 5 weight %; and the obtained three coating films are simultaneously cured by heating. However, the correlation between the fact that the weight loss (eliminated component) of the electrodeposition coating film is small when the electrodeposition coating film is cured by heating, and the corrosion resistance and the finish of the multilayer coating film, is not clear.